1. Field of the Invention
The present invention relates to a system for transmitting a beam in the air, and in particular to a system for transmitting a beam in the air, which is based on the transmitting method using coherency of a laser beam.
2. Description of the Related Art
As electronic devices have been made more and more wireless, a system for transmitting a medium like light or a microwave in the air has been more requested. Further, considering that the needs for information about a moving image may go high, it is essential to speed up such a system and increase the volume of the information to be transmitted by the system (as an example of a digital signal, a transmission speed=10 to 600 Mbps: the number of bits to be sent or received as a digital signal per second). Of the transmitting media, transmission of a beam in the air may be applied in the wide range of a small-scaled wireless LAN, AV equipment and image communication equipment, because the transmission is not restricted by the laws unlike the transmission of microwaves. It is expected that the beam may convey a greater amount of information by transmitting in the air instead of through other media.
In the conventional system for transmitting a beam in the air for civilian use, which may be used in any living place, a system is provided to have a light-emitting diode as a light source. Such a system applies to a remote controller for electric household appliances or a domestic LAN operating at a relatively low speed (600 Kbps or lower). The summary of the conventional system for modulating a beam in the air has been discussed in detail in pp. 94 to 100 of "Current State and Feature of Optical Communication Technique" edited by Electric Communication Council in 1987. The system configuration of this example is shown in FIG. 13.
A transmitting side is arranged to have an LED 111 and an LED driver 112 and to modulate an intensity of the ray of light sent from the LED 111 according to the signal to be transmitted and emit the modulated light in the air 113. On the other hand, the receiving side is arranged to have a lens 114, a light detection element 115, and a signal processing circuit 116. In the receiving side, the transmitted intensity-modulated light is condensed on a light receiving surface of the light detection element 115 through the effect of the lens 114. The light intensity is converted into an electric signal. The electric signal is amplified and shaped by the signal processing circuit 116 for obtaining a desired signal. An example of a light signal waveform used in the conventional system for transmitting a beam in the air is shown in FIG. 14. The signal is represented as a time change of the intensity of light propagated through the air. A detecting mechanism operates to directly detect the intensity of the light signal. That is, this conventional system may be referred to as an intensity modulating (IM modulation)--direct detecting system with light as a medium.
In the high-speed optical transmission, the time width (pulse width) of a transmitted light pulse is made narrower according to the signal. This results in inevitably reducing a light power per one pulse. The minimum acceptance sensitivity (number of photons per one pulse for securing the lowest S/N ratio required for demodulating a signal in the receiving side) in the IM modulation--direct detecting system shown in FIG. 13 is about 10.sup.4 to 10.sup.5 photons/pulse. Assuming that the LED 111 has a luminous wavelength of 800 nm, a light output power of 200 mW, an emitting angle of .+-.60.degree., a distance between the transmitting side and the receiving side is 5 m, a condensing efficiency is 5 times, and an effective acceptance area of the light detection element is 2 mm, the possible transmitting speed is at most 1Mbps. Such a low transmitting speed makes it impossible to transmit a high-definition moving image correctly from a visual point of view if a data compressing technique may apply to the transmitting system.
To make the conventional transmitting system operate at high speeds, it is necessary to increase the light output power of the transmitting side. To realize the transmitting speed of about 10Mbps or more, the required light output power is ten times as large as the current intensity (2 W). The LED cannot cope with such a high light output power. That is, the prior art has no capability of increasing the transmitting speed and distance of a beam in the air.
Another problem about the transmission of a beam in the air entailed in the prior art is that background light (skylight or room lighting) is incident to the light detection element as noises. Under such a circumstance, the conventional transmitting system is not usable. This results from the fact that the LED for emitting a signal beam used in the conventional system for transmitting a beam in the air emits an incoherent beam and the emitted beam lacks monochromaticity. This is because since the light (background light) in the natural world is also incoherent light, the background light existing in the same wavelength-area as the signal beam cannot be separated from the signal beam and since the signal beam has a relatively wide spectrum, the power of the background light mingled as noises is inevitably made larger.
As described above, the conventional transmitting system for civilian use of the IM modulation--direct detecting type has the following problems:
(1) It cannot realize the large transmitting capacity necessary to transmit information such as a high-definition moving image by transmission of a beam in the air. PA1 (2) Where strong background light such as skylight exists, the signal beam is offset by the background light. Hence, the signal beam cannot be practically demodulated. PA1 a transmitting device having means for generating a signal beam by modulating the wavelength or the phase of the coherent beam according to a signal to be transmitted and means for emitting the signal beam as a spherical divergent light wave; PA1 a receiving device having means for generating a coherent locally oscillated beam, photo detection means for mixing the signal beam with the locally oscillated beam and receiving the mixed beam, frequency discriminating means for detecting a light frequency difference between the signal beam and the locally oscillated beam from the output sent from the photo detection means, and means for reproducing a signal from the output sent from the frequency discriminating means; and PA1 relative wavelength tuning means for controlling the wavelength of the locally oscillated beam or a reference wavelength of the signal beam to hold the difference between a reference wavelength of the signal beam and a wavelength of the locally oscillated beam at a predetermined value, the tuning means being provided in any one of the receiving device and the transmitting device.